The present invention relates to an improved patellar prosthesis and method of making the same.
Various prosthetic devices for replacement of the articulating surface of the patella or knee cap have been designed for implantation in cases where replacement of the natural articulating surface of the patella is indicated. Generally, the posterior surface of the patella is resected and replaced by an artificial articulating surface.
Various patellar prostheses are known in the art and available commercially to surgeons. Examples of such are the designs shown in U.S. Pat. No. 4,240,162 issued to Devas on Dec. 23, 1980 in which a two-part snapped-together design is disclosed. Commercially available implants are shown, for example, in Publication No. 322-688T of Dow Corning Wright Corporation, 5677 Airline Road. Arlington. TN 38002 entitled "Whiteside Ortholoc II Patella Implants". Similar designs having a metal backing within a polymeric surface portion are shown, for example, in a publication by Howmedica, Inc., Orthopedics Division, 359 Veterans Boulevard, Rutherford, NJ 07070 identified as No. H2050, dated 3/83, entitled "The PCA Revision Total Knee System" at page 4. Such prostheses have been known to fail by virtue of separation of the polymeric articulating surface from the metal backing.
In light of these various designs, a need has existed for an improved patellar prosthesis having resistance to shear forces experienced during articulation of the patellar implant against an articulating surface of a femoral prosthesis. The present invention provides such increased resistance by a design in which shear forces imposed against the side of the patellar implant are, in part, converted to compression forces. The invention further provides a means and method of molding the polymeric articulating surface onto the metal backing in such fashion as to improve the mechanical locking, and thus resistance to wear, between the components of the prosthesis.
Briefly summarized, the present invention provides a composite patellar prosthesis for implantation in the human body to replace the articulating surface of the patella which includes a polymeric articulating surface portion, and a dome shaped metal backing which has an anterior surface adapted to be attached to the posterior surface of a resected patella, preferably by means of integral attachment pegs. The dome shaped, or otherwise centrally elevated posterior surface is adapted to engage the polymeric articulating surface portion which is molded under heat and pressure to conform to it. The metal backing portion is provided with at least one, and preferably several apertures extending therethrough which have a greater cross-sectional area on the anterior side than on the posterior side. These apertures are each adapted to fixedly receive an anchoring peg integral with the anterior surface of the polymeric articulating surface portion, the pegs being formed under heat and pressure to precisely fill the apertures. It is also preferred that the anterior surface of the polymeric articulating surface portion is coextensive in size and shape with the metal backing portion is molded to conform to the contour thereof. The articulating surface portion has a generally convex posterior articulating surface. The apertures through the metal backing are greater in area on the anterior side than on the posterior side so that the pegs firmly lock the two parts of the composite together. The anterior side may, for example, be counterbored or the apertures may be tapered or flared so as to be of greater size on the anterior side.